1. Field of the Invention
The present invention relates to a system and method for determining position of a user terminal or other communication equipment based on time of arrival measurements in a wireless environment.
2. Description of the Related Art
Measurements of times of arrival (TOA) for signals from a set of wireless base stations can aid in determining a user's position or location. For example, an LTE (long term evolution) standard receiver can determine its location (or positioning) based on LTE signaling in a way that can replace or supplement GPS or WiFi-assisted positioning strategies.
The LTE positioning protocol, described for example in ETSI TS 136 355 version 10.0.0, which is referenced in release 10 of the LTE specification, embeds positioning reference signal (PRS) subcarriers into designated orthogonal frequency division multiplexing (OFDM) symbols over specified time intervals, sometimes called positioning occasions. The user equipment (UE) may measure the time of arrival (TOA) of PRS subcarriers from each accessible base station (which the LTE specification calls the eNodeB). The user equipment preferably measures at least one reference signal time difference (RSTD) between two different eNodeBs (one called the reference and the other called the neighbor). The reference signal time difference is related to the established measure for observed time difference of arrival (OTDOA) described in the LTE positioning protocol.
The reference signal time difference measurement is simple in concept. In practice, the measurement of any TOA from an eNodeB can be unreliable because of errors due to the severe multipath environment in wireless networks and the typically low signal-to-noise ratio (SNR). The increased density of base stations and users in an LTE network also increases the potential for measurement errors. Determining the position of user equipment proceeds by measuring the TOA of the first path from each eNodeB of interest followed by determining the reference signal time difference (RSTD) between pairs of designated eNodeB base stations using the respectively measured times of arrival at the user equipment. Depending on the particular configurations specified in the standard, the TOA and RSTD measurements may be made over a specified number of base stations and different corresponding combinations of RSTD measurements between ones of the set of base stations.
The difficulties that arise in measuring TOAs relate to identifying the first path arriving at a user equipment terminal (UE) from any designated eNodeB. It is common for a wireless channel's impulse response to consist of a small plurality of paths at varying amplitudes and delays relative to a first path. The strongest path in a wireless channel's impulse response may not be indicative of the true delay, since the first path may be of lower amplitude than the strongest path. In addition to the fact that the first path might not have the greatest amplitude, identification of the first path can be complicated by the likelihood of falsely identifying paths due to correlation with interference, high levels of noise, or both.
LTE's fundamental modulation scheme to transmit bits over the air uses OFDM. That is, bits are generated by applying quadrature amplitude modulation (QAM) to each active subcarrier that makes up an OFDM symbol. In practice, an LTE OFDM symbol may have 1024 time samples representing 600 active subcarriers out of a maximum of 1024 subcarriers. Each subcarrier may be assigned a function at the receiver, such as transmitting bits known a priori to the receiver and thus enabling different calculations. These calculations may include channel impulse response (CIR) estimation and positioning-related measurements.
FIG. 1 provides a functional block diagram of apparatus for determining position using observed time difference of arrival (OTDOA) based on the reference signal time difference (RSTD) measurement specified in LTE. The illustrated user equipment receiver 110 receives a plurality of OFDM symbols from two base stations 101, 103. Receiver 110 may use one or more antennas to receive the symbols. FIG. 1 illustrates the position determining functionality using as an example signals received from two base stations 101, 103 with the receiver 110 using a single antenna, which is the minimum configuration for an RSTD measurement. This configuration can be extended to a greater number of base stations and a greater number of user equipment antennas.
Because user equipment receiver 110 is compliant with the LTE standard, the receiver can process received OFDM symbols to provide best estimates of the transmitted bits. Such a receiver 110 can identify the first path using one or more first path identification (FP-ID) modules 130, 140, which are responsive to subcarriers assigned to calculate positioning information. Each first path identification module 130, 140 is responsive to information 132, 142 provided by the user equipment receiver 110 about the subcarriers to be used for positioning measurements. For example, the information may be stored within tables in non-volatile memory.
The first path identification units 130, 140 identify the respective first path for the received OFDM symbols from a known eNodeB. The reference signal time difference (RSTD) measurement typically is based on a predetermined duration of OFDM symbols to achieve the desired accuracy. In LTE, this may be over at least one sub-frame of OFDM symbols, which is specified to be fourteen OFDM symbols.
The output from each first path identification module 130, 140 is the time of arrival (TOA) at the user equipment of a signal from the corresponding base station. Generally, in LTE, the RSTDk,j between base stations indexed as k and j is determined asRSTDk,j=TOAk−TOAj.  EQ. 1FIG. 1 shows that module 150 of the receiver 110 provides as its output 152 the equation 1 reference signal time difference computation. This output RSTD0,1 152 is the output 134 of first path identification module 130 minus the output 144 of first path identification module 140.
The calculation of RSTDk,j is simple given a reliable estimation of the TOAk and TOAj, knowing the structure of the signal received at the first path identification FP-ID module 130, 140. The standard, such as the LTE standard, specifies the structure of the symbol, which can be generalized as shown in FIG. 2. Modern wireless systems that transmit from one source, such as a base station, to multiple users in the coverage area, require the transmission to be subdivided into “subchannels.” This is not much different in concept than FM or AM radio transmission; however, wireless transmission has a goal of very high bits/Hertz for a given spectrum. In the case of modern wireless technology, specifying channels can be achieved using orthogonal schemes, which include OFDM and code division multiple access (CDMA). In the near future the wireless standards may increase capacity by using quasi-orthogonal channels achieved in myriad spatial and temporal strategies.
FIG. 2 simplifies the explanation of the signals involved in an observed time difference of arrival (OTDOA) measurement by showing a method that assumes orthogonal channelization. That is, while orthogonality is retained, the crosstalk between channels is kept to insignificant levels.
The FIG. 2 horizontal axis 201 represents time, qualitatively representing the time occupied by received symbols. FIG. 2's vertical axis shows a second channel dimension such that FIG. 2 qualitatively shows channels as having no overlap. The vertical axis channel separation can represent segments of frequency, as in the case of OFDM, or the indexing of different codes in CDMA. For example, in the present LTE standard, the segmentation in the frequency axis can represent 15 kHz of bandwidth for a subcarrier, with an OFDM symbol possibly consisting of up to 600 active subcarriers out of 1024 total subcarriers in one symbol. This is only an example and other allocations are known. Thus, for example, the extent of each square in FIG. 2 can represent 15 kHz (y-axis) by 71.4 μs (x-axis) in the frequency-by-time grid. The value 71.4 μs is determined by the 1000 μs duration of an LTE sub-frame divided by the number 14 of OFDM symbols specified as making up an LTE sub-frame. In LTE terminology, each 15 kHz (y-axis) by 71.4 μs (x-axis) block in the grid is called a resource block (RB).
The following discussion of FIG. 2 focuses on OFDM transmission, but it should be appreciated that FIG. 2 can equally illustrate other transmission systems. For example, FIG. 2 can illustrate other orthogonal schemes such as CDMA transmission as well as quasi-orthogonal transmission strategies such as those proposed for next generation wireless (5G). The orthogonal or quasi-orthogonal transmission strategies might be used for subchannels or for signaling related to observed time difference of arrival (OTDOA) measurements, among other transmission strategies.
To allow for user equipment terminals to determine position by computing OTDOA, certain wireless standards assign subcarriers in the grid to be used for determining position or accomplishing OTDOA functionality. To simplify this discussion, exemplary OTDOA subcarriers are designated as “location pilots” (LP) 212, 214, 216 in FIG. 2. The term pilot is used to denote a subcarrier with a known transmit modulation at the receiver. These pilot subcarriers are in contrast to data subcarriers, which have unknown modulation characteristics because they are encoded with unknown information bits. This pilot scheme allows a compatible terminal to accomplish various measurements. User equipment terminals generally need to estimate the channel impulse response (CIR) and other parameters for successful reception and demodulation of OFDM symbols. Consequently, the grid shown in FIG. 2 likely contains other subcarriers designated as pilots. These persistent pilots are denoted as estimation pilots (EP) and are indicated as 221, 223, 225 in FIG. 2. In LTE, these locations, and EP modulating bits, are known at the receiver because they are dictated by the LTE specification.
While the user equipment is guaranteed to receive the location pilots at the time of a request for OTDOA measurement, the number of estimation pilots present will vary by configuration of the user equipment in the network. In the context of LTE, the location pilots are designated as positioning reference signals (PRS), and the estimation pilots are designated as common reference signals (CRS). In the LTE scheme, the user equipment receiver will always receive at least one set of CRS subcarriers in a sub-frame, and possibly may receive additional sets of subcarriers. It is reasonable to consider that a user equipment receiver can receive up to four CRS sets in a sub-frame. In the nomenclature of LTE, these sets are called “antenna ports.” Normally, CRS subcarriers are used for channel impulse response (CIR) estimation, among other parameter estimations. Another property of the grid shown in FIG. 2, as it applies to LTE, is that the CRS and PRS are distributed on the 600-by-14 grid, and need not be contiguous in time or frequency. FIG. 2 shows arbitrarily positioned location pilots and estimation pilots consistent with this observation.